746 



ECOLOGY 



ranean, but not when aerial (figs. 1070, 1108). The root tubercles of legumes (see 

 p. 787), which are known to be the direct result of bacterial infection, resemble tubers 

 not alone in the replacement of elongation by lateral enlargement, but also in the 

 great accumulation of food and water in the hypertrophied cells. Again, in many 

 fungus and insect galls (as in the cedar apple and in cynipid oak galls), elongation 

 is checked, while diametral increase and the accumu- 

 lation of food and water are greatly stimulated; the 

 food may accumulate even in definite carbohydrate and 

 protein layers. Thus root tubercles, galls, and tubers 

 agree in all essential structural features; root tubercles 

 and galls admittedly are due to the influence of out- 

 side organisms, and it seems fairly certain that the 

 same is true of tubers. 



If a gall is defined as a structural modification due 

 to a foreign organism, then potato tubers and the 

 gametophytes of Lycopodium and Bolrychium may be 

 classed as galls, if the fungus theory is confirmed. The 

 fact that tubers and root tubercles are advantageous, 

 as is not true of many fungus galls and of most insect 

 galls, is of no significance from the standpoint of cau- 

 sation, and would seem unimportant in classification; 

 the harmful root tubercles caused by nematode worms 

 and the beneficial root tubercles caused by bacteria are 

 remarkably alike in form and origin, and may well be 

 classed as similar structures. 



Bulbs, as well as tubers, commonly are infected 

 with fungi, which, therefore, may have formative sig- 

 nificance, though the case is much more doubtful than 

 with tubers. While bulbs agree with tubers in possess- 

 ing shortened stems, they differ in that lateral stem en- 

 largement is replaced by hypertrophied growth and by 

 food accumulation in the scale leaves. Arrhenaiherum 

 bulbosum has been shown to be merely a bulbous 

 form of A. elatius, resulting from bacterial infection. 



Fig. 1070. — The sub- 

 terranean tuberous game- 

 tophyte (g) of Lycopodium 

 annotium, bearing a young 

 sporophyte (.s), whose 

 aerial portion has numer- 

 ous awl-shaped foliage 

 leaves (/) in many orthos- 

 tichies. 

 — From Bruchmann. 



The nature of the fungal influence. — In orchids and in the potato, 

 tuberization has been induced in concentrated solutions (as of sac- 

 charose or glycerin) without the agency of fungi ; similarly in the onion, 

 bulb formation has been induced in sterilized cultures. When radishes 

 are grown in concentrated glucose solutions, the root becomes suberized, 

 somewhat resembUng a potato tuber, and starch accumulates instead 

 of sugar. Apparently, then, tuberization may result when the osmotic 

 pressure in the culture medium is high, though this does not appear to be 

 the sole factor, since solutions of glucose and glycerin of equal pressure 

 give different results. Certainly for starch formation, and perhaps for 



